In recent years, in the fields of electronic devices, the technical field called printable electronics has attracted attention, in which a very fine wiring of a micrometer size is directly formed by an inkjet printing method or other printing methods without a need of patterning of a wiring or a protective film by light exposure. Fine particles of gold or silver were mainly used at the beginning of the development of this technique, but gold has a problem of the cost, and silver has problems about the electromigration and the corrosion resistance for, for example, corrosion due to a sulfide gas. As a means for solving these problems, copper materials have attracted attention. Copper materials exhibit high conductivity equivalent to gold or silver and are considerably excellent in respect of the electromigration, as compared to silver, and further have excellent corrosion resistance.
Gold and silver, which are a noble metal, have properties such that they are relatively unlikely to be oxidized. For this reason, when a dispersion of metal fine particles of gold or silver is prepared, it is easy to maintain the dispersion while preventing an oxide film from forming on the surface of the metal fine particles contained in the dispersion. In contrast, copper has properties such that it is relatively likely to be oxidized, and this tendency is further remarkable particularly for the copper fine particles having a particle diameter as small as 200 nm or less due to the size effect and specific surface area. When a dispersion of copper fine particles is prepared, the copper fine particles contained in the dispersion become in a state in a short period of time such that the surface of the copper particles is covered with an oxide film, and further the oxide film on the surface of the copper particles increases in thickness with the passage of time, so that the portion occupying almost all the particle diameter of the copper fine particles is often converted to the copper oxide film. Further, 200 nm or less copper fine particles are in a state such that the surface of the particles is extremely highly active, and, even when subjected to a method in which heating or calcination is performed in an inert atmosphere of, for example, nitrogen gas or under vacuum conditions, oxidation may proceed due to a very small amount of oxygen present in the atmosphere to inhibit sintering of the copper fine particles with one another. Further, when reducing calcination is performed using, for example, hydrogen gas at the final stage of calcination, the increase of the oxide film during the calcination may cause marked volume shrinkage upon reducing the film, leading to a lowering of the calcination density.
Meanwhile, one of the reasons why the technique using metal fine particles has attracted attention resides in melting point depression due to a size effect. Taking gold as an example, the melting point depression due to a size effect has been reported as follows. Gold in the form of a simple substance has a melting point of 1,064° C. When gold is in the form of particles having a particle diameter of about 2 nm, the melting point is reduced to about 300° C. which is a temperature at which electronic materials and others can be used. However, when gold is in the form of particles having a particle diameter of more than 20 nm, almost no melting point depression is recognized. From the above, single-nanometer-sized metal fine particles having a particle diameter of about 2 nm can be satisfactorily expected to suffer melting point depression. With respect to the copper fine particles, however, a surface protecting agent for preventing oxidation is indispensable. Considering the specific surface area of the copper fine particles, the amount of the surface protecting agent required for the copper fine particles is several times or more the volume of copper, and the surface protecting agent in such a large amount causes marked volume shrinkage during the sintering, making it difficult to obtain a sintered material having a high density. For removing this disadvantage, a method has been known in which single-nanometer-sized particles are formed from a metal oxide in a reducing atmosphere at the sintering stage, and sintered at a temperature of about 300 to 400° C. utilizing the melting point depression due to a size effect. In addition, a method has been proposed in which, like a flux effect of a solder, an oxide film covering the surface of fine particles is removed by a fluxing agent, such as an organic carboxylic acid, so that the reduced metal surface is exposed, followed by sintering (see, for example, Japanese Unexamined Patent Publication No. 2013-047365).
When the copper fine particles are applied to printable electronics, a paste is prepared from the copper fine particles and then supplied. Therefore, copper fine particles exhibiting a monodisperse particle diameter distribution are prepared so as to obtain a copper paste material having excellent dispersion stability. With respect to the method for producing metal fine particles or metal oxide fine particles having a uniform particle diameter, several proposals have been made. For example, with respect to a liquid phase synthesis of metal fine particles, reference is often made to the LaMer model representing the relationship between the solubility of the solute which constitutes a metal nucleus and the time. According to this, when the rate of formation of metal nuclei having a low solubility is too fast, growth of particles occurs in accordance with an aggregation mechanism, so that the growth of crystal nuclei is disadvantageously unsatisfactory to cause particles in an aggregate form. For solving this problem, a method of controlling the rate of formation of metal nuclei which are a solute has been made. For example, by permitting a material needed for the growth of particles to be gradually emitted from a reservoir (solid or metal chelate), the degree of supersaturation of the solution is controlled to suppress new nucleation during the growth of particles, so that the nucleation period and the particle growth period are separated so as to allow only the nuclei formed early in the initial stage to grow, making it possible to form monodisperse particles. As a method of selecting a reservoir for supplying the solute during the growth of particles, a solid or complex compound having a satisfactorily low solubility or dissolution rate is selected.
In connection with the above, a technique in which a complex compound derived from copper formate is subjected to thermal decomposition to produce copper fine particles has been known. Copper formate has a decomposition temperature of about 220° C., but copper formate having a complex structure can be reduced in the decomposition temperature. For example, Japanese Unexamined Patent Publication No. 2011-032558 has proposed a method in which, using a complex compound of an amino alcohol which functions as a bidentate ligand, the complex compound is subjected to thermal decomposition at 100° C. to produce metal fine particles. Japanese Unexamined Patent Publication Nos. 2008-013466 and 2008-031104 have proposed a method in which, using a complex compound of an aliphatic amine which functions as a monodentate ligand, the complex compound is subjected to thermal decomposition at 120° C. to produce metal fine particles.
Further, a method has been made in which the metal nuclei incorporated into the growing nuclei in a micro-reaction field using a surfactant are restricted to control the particle diameter. For example, there has been proposed a method for producing metal or metal oxide fine particles by a reversed micelle method in which nanometer-sized water droplets stably dispersed in an organic solvent using a surfactant are used as a reaction field (see, for example, Japanese Unexamined Patent Publication Nos. Hei 08-143916 and 2009-082828 and Japanese Patent No. 3900414).